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Old 3rd December 2008, 10:27 AM   #1
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Default Speaker ground issue

Impressed with level of knowledge here, I hope this question isn't beneath y'all! Can anyone help a newb??

I'm trying to connect this (Rocketfish Universal Wireless Rear Speaker Kit )

Click the image to open in full size.

to a powered set of PC speakers Altec Lansing ATP3's.

The PC speakers input is 3.5mm male audio (designed to go into the line out of a sound card).

I got an RCA to 3.5mm male ala:

Click the image to open in full size.

and chopped off the RCA ends, and shoved them into the speaker outputs on the receiver. THen used a double 3.5mm female coupler to connect to the Altec Lansings.

Problem is the sound comes out 90% one speaker. If I leave one of the grounds unplugged, it goes pseudo stereo i.e. left only comes out of left speaker but right comes out of both in mono.

I assume it's b/c that cable goes to a common ground (4 wires -> 3 wires).

How do I do this? I'm thinking of getting an isolator ala:
Click the image to open in full size.

and a couple adaptors any thoughts???

TIA!
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Old 3rd December 2008, 01:57 PM   #2
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First of all, your trying to connect a power amplifier into another power amplifier. To complicate the issue, the amplifier in the Rocketfish is a BTL class-d amplifier. You cannot connect the output "grounds" together.

There are a few different ways you can get this to work, but my brain isn't up to the task of explaining it all right now. If someone else doesn't chime in with a viable solution I will try and post one later.
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Old 3rd December 2008, 03:58 PM   #3
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"You cannot connect the output "grounds" together."

So I think that's exactly what that cable I have pictured does, hence my thought on the isolator.

Additional detail is that the Rocketfish says it will power 4 to 16 ohm speakers and the Lansing input impedence is 10 KOhm. Seems I'm off by a factor of 1000 so seems like that needs to be addressed as well?
I guess it would make sense that a powered amp (the Lansings) are much "stiffer" than a regular unpowered speaker.
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Old 3rd December 2008, 04:53 PM   #4
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Not sure what you mean by "stiffer".

Just to clarify something, the negative outputs of the Rocketfish are not "grounds" at all, they are positive outputs that are out of phase with the other positive outputs (the ones marked +).

Some info on bridged outputs....

http://www.bcae1.com/bridging.htm
http://en.wikipedia.org/wiki/Bridged...dged_amplifier
http://sound.westhost.com/project14.htm

Here are three ways to do what you want (easiest to hardest) without going into too much detail.

1) Buy a high to low level converter. You will need to put a ~16R 10w resistor across each channel input to properly load the output filter of the class-d amp.

http://cgi.ebay.com/ws/eBayISAPI.dll...6610%26fvi%3D1

2) Put a ~16R 10w resistor across the outputs of each channel. Take a >10uF capacitor and connect one side to the + output of each channel. On the opposite ends connect your signal cable that goes to the powered speakers. Now take the ground of the signal cable and connect it to the POWER ground of the Rocketfish (NOT the output negatives).

3) Intercept the line level audio signal inside the Rockefish before it goes to the input of the class-d amp and route it to the powered speaker inputs.

EDIT: I forgot to mention the most obvious cure for your problem; get a pair of regular bookshelf speakers and use them with the Rocketfish.
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Old 4th December 2008, 10:37 AM   #5
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Thanks a ton anon1!!

Picked up an adjustable line output converter (option 1) which I got at a car audio shop, did the trick. Saved myself from having to spring for new speakers. Plus the ATP3 has more juice than the receiver.

It sounds not too bad, but problem is when my 802.11 starts up I get interference... even when I switch my network's channel. I'll have to look into remedies for this, if there are any.

By saying stiffer I guess I'm just demonstrating my sophomoric grasp of electronics. I'll shut up.

This problem is something I'm surprised wasn't out there that I could find. I really searched. If anyone can find it somewhere else I'd be curious where and how (search string) they got there. I was even just searching for "stereo speaker out to pc speakers" or something like that.
Of course the whole wireless is unnecessary (I just wanted to include the exact specs of my setup).


Thanks again!
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Old 4th December 2008, 10:37 AM   #6
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And BTW (another post b/c I'm still modded)

"Just to clarify something, the negative outputs of the Rocketfish are not "grounds" at all, they are positive outputs that are out of phase with the other positive outputs (the ones marked +)."

That is something everyone should know about speaker outs, I'm glad I do now!
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Old 4th December 2008, 01:20 PM   #7
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Quote:
Originally posted by lowfrequency
That is something everyone should know about speaker outs, I'm glad I do now!
Only bridged outputs; single ended amplifier outputs are referenced to ground (meaning the negative lead is actually ground).
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